1. Field of the Invention
The present invention relates to soft X-ray generation, and more particularly, to a method and apparatus for generating soft X-rays for semiconductor manufacturing.
2. Discussion of the Related Art
In a conventional process, a pattern from a mask is reduced, projected and transferred onto a silicon wafer using visible or ultraviolet light during the manufacturing of semiconductor integrated circuits. The silicon wafer is coated with a photoresist compound that is sensitive to selected wavelengths of light. As technological advances necessitate the use of increasingly more complex and finer patterns, the diffraction limit of light sources is quickly being approached. Thus, the use of soft X-rays with wavelengths shorter than the wavelengths of ultraviolet light, i.e., wavelengths of about 13 nm or about 11 nm, is becoming necessary to achieve required pattern resolution.
A laser plasma X-ray source (LPX) may be used to produce soft X-rays with a wavelength of about 13 nm or about 11 nm. In the LPX, electrons are stripped from the atoms of a target material by an intense electric field when a pulsed light with an irradiation intensity exceeding about 10.sup.10 W/cm.sup.2 is focused and directed onto the target material. The target material is converted into a plasma, and soft X-rays radiate from the plasma. The soft X-rays have an extremely high brightness. The LPX apparatus is very compact when compared to other light sources capable of producing soft X-rays (for example, a synchrotron facility). Thus, use of the LPX to generate soft X-rays for semiconductor wafer exposure is preferred. LPX's may also be used for X-ray microscopes, as well as other applications.
The generation of debris is a serious problem in the LPX. When a solid target material, such as metal, is irradiated with laser light, ions and atoms which form the plasma are displaced by the rapid expansion of the plasma. Portions of the target material located in the vicinity of the plasma are scattered into the surrounding space. Optical elements, e.g., multi-layer film mirrors, thin-film filters, etc., are frequently installed in the vicinity of the plasma in order to shape the beams of generated soft X-rays. Thus, debris are deposited on the surfaces of the optical elements. This results in a deterioration of their optical performance (for example, reflectivity or transmissivity, etc.). Plasma is typically generated over a long period of time (often many months) at a high frequency (e.g., 1 kHz or greater) during soft X-ray exposure of semiconductor wafers. Accordingly, a large quantity of debris is generated over time. Thus, a reduction in the generation of debris is of great interest.
Kublak et al., in U.S. Pat. No. 5,577,092, show an LPX which uses a gaseous target material to solve the problem of debris generation. In their approach, a gas is directed through a nozzle at a supersonic velocity into a vacuum vessel. Clusters of gas molecules are formed through the resulting adiabatic expansion. Plasma is generated by irradiating these clusters of gas molecules with laser light. Since the target material is a gas, there is no deposition of the target material on the surfaces of the soft X-ray optical elements, even if the gas diffuses into the surrounding areas after plasma formation. Accordingly, there is no drop in the optical performance of these elements. Thus, it is reported that the useful life of the mirrors in such a soft X-ray optical system may reach 10.sup.9 shots.
Richardson et al, in U.S. Pat. No. 5,577,091, show an LPX which uses fine water droplets as a target material. In this method, the fine water droplets are directed into a vacuum vessel. Plasma is formed by irradiating the fine water droplets with laser light. The fine water droplets evaporate at the time of plasma formation. Fine water droplets that are not irradiated and are scattered by the plasma (and adhere to the optical elements) are easily removed. Thus, the degradation of performance as a result of debris generation and scattering is reduced.
However, the prior art soft X-ray sources have the following problems. First, the X-ray spatial intensity distribution is frequently non-uniform and asymmetrical, resulting in uneven exposure of a semiconductor wafer. This often results from the plasma region being nonspherical and asymmetrical. Second, structural components needed to support the nozzle and other elements within the vacuum vessel generate shadows, which further reduce the quality of the X-ray intensity distribution.